These are sophisticated equipment and highly capital intensive. Hence they can be considered more as part of the extraction operations and not as purely field level post harvest infrastructure. More skilled manpower is required for operating these machines and they are not amenable to field level maintenance.
Microwave is an electromagnetic radiation which has wavelengths shorter than radio waves and longer than infrared radiation. The frequencies allocated for industrial drying are 896 MHz and 2450 MHz. Most molecules, although electrically neutral, have an asymmetrical distribution of electrons and may be electrically positive at one end and negative at the other. Water molecule also has a dipolar property because the electrons are more concentrated on the oxygen atom within the molecule. In an electric field, the molecules align themselves in a specific direction and when the field is removed, it returns again into its random orientation. A microwave radiation creates a rapidly pulsating electrical field, which rapidly changes the molecular orientation back and forth. The resultant molecular vibration produces heat. This is the principle for microwave heating and drying. In microwave dryers, microvave radiation is generated by magnetron type vacuum oscillators.
In continuous microwave dryers, the Stevia leaves are passed through a tunnel on a conveyor belt. The conveyor belt and the construction material of the inside of the tunnel is made of microwave resistant material. In the tunnel, the leaves are exposed to a series of microwave sources and the water molecules in the fresh Stevia leaves are heated up to the boiling point of water. The water in the leaves is thus evaporated at high temperature and the water vapour is removed from the tunnel by air blowers. The microwave energy, belt speed and the leaf feed speed is precisely controlled by microprocessors/computers so that the leaves are dried properly and are not exposed to unnecessarily high temperature. The industrial microwave dryers can give a throughput of few tons per hour.
The tray dryers are actually a large rectangular double walled chamber with thermal insulation between the walls. The walls are made of sheet metal supported by a metallic framework. The chamber is provided with racks for placing shallow trays in it. Otherwise, the tray can be arranged on a rack on wheels and the entire rack can be pushed into the dryer cabinet.
The leaves to be dried, are thinly spread on the trays and the trays are loaded in the chamber. The chamber is provided with an air heater, which heats up the air within the chamber and an array of fans/blowers which guides uniform air flow on all the trays. The hot air picks up moisture from the leaves and the moisture laden hot air is discharged from the chamber by exhaust blowers. Generally, 80% to 90% of the hot air is circulated back in the chamber and 10% to 20% is discharged outside to maintain energy efficiency of the equipment. The temperature within the chamber is controlled by thermostats.
The hot air within the chamber may be generated by electrical heaters of by oil/biomass burners outside the chamber. In case of electrical heating, the thermostat switches the heater on and off to control the temperature. In case of biomass heaters, the rate of entry of hot air may be regulated by the thermostat by electromechanically opening and closing damper valves.
Belt Mesh Dryer
A Belt dryer is a device designed for the particularly gentle thermal treatment of product. The wet product is continuously and evenly applied through an infeed chamber onto a perforated belt. The belt, predominantly in horizontal position, carries the product through the drying area which is divided into several sections. In these cells drying gas flows through or over the wet product and dries it. Each cell can be equipped with a ventilating fan and a heat exchanger. This modular design allows the drying temperatures to be controlled separately in the different sections. Thus, each dryer cell can be individually controlled and the drying air flow can be varied in each cell. In addition, the speed of the conveyor belt can be varied what gives an additional parameter for setting of drying time. The cells can be heated directly or indirectly, and all heating media, such as oil, steam, hot water or hot gas can be used.
The amount of air required to discharge the exhaust vapor from the belt dryer is sucked in by a radial fan and fed to the appropriate zones of the belt dryer. Due to the arrangement of the air inlet and outlet ducts to and from the appropriate zones, the drying air is passed repeatedly through the product layer until it has the optimum vapor content before leaving the dryer.